1. Field of the Invention
The present invention includes an image sensor including a temperature sensor and operable to use the temperature sensor compatibly with an electronic shutter pulse. The present invention also includes a method of compatibly using the temperature sensor and the electronic shutter pulse.
2. Description of the Related Art
Performance of an integrated circuit, such as an image sensor, can be dependent on the temperature of the integrated circuit. As one example, dark current inside an image sensor is highly temperature dependent. The dark current increases with an increase of temperature of the integrated circuit and higher dark current degrades the performance of the image sensor. Higher dark current impacts the dynamic range and the dark reference level of the image sensor and can cause various defects in captured images. The image sensor is also susceptible to permanent damage if the temperature becomes too high.
The image sensor can include a temperature sensor, such as a temperature diode, for measuring the temperature of the image sensor. The measurements from the temperature diode can be read by a reading component, such as an analog-to-digital converter, and a processor connected to the reading component can control a thermoelectric cooler coupled to the image sensor based on the temperature measurements.
When voltage is applied across the temperature diode to forward bias the diode, current flows through the diode. The relationship between the voltage across the diode and the current through the diode is temperature dependent. In other words, at the same voltage, the current increases with the temperature. Likewise, at the same current, the absolute value of the voltage decreases with the temperature. When the relationship between voltage across the diode and the current through the diode is calibrated for the image sensor, the temperature of the image sensor can be determined by reading one of these parameters while setting the other parameter at a constant.
One advantage in some types of image sensors, e.g., an interline transfer image sensor, is the ability to apply a global reset to an image sensing region of the image sensor by applying a high voltage pulse to the substrate of the image sensor to drain away all charge in photodiodes of the image sensing region prior to image capture. The high voltage pulse is referred to as an electronic shutter pulse. However, when voltage associated with the electronic shutter pulse is sufficiently high, e.g., above 17V, substrate punch-through occurs, which increases the voltage across the temperature diode. Since the temperature measurement from the temperature diode is dependant on the relationship between the voltage across the diode and the current through the diode, the voltage increase across the temperature diode due to the substrate punch-through from the electronic shutter pulse disadvantageously alters the temperature measurement from the temperature diode.
In other words, the electronic shutter pulse causes substrate punch-through at the diode and corrupts readings from the temperature diode, thus making the temperature diode and the electronic shutter pulse incompatible features. The voltage increase across the temperature diode from the electronic shutter pulse can also cause damage to the reading component. There remains an opportunity to design a circuit that can determine the temperature of the image sensor without corruption from the application of the electronic shutter pulse.